SPIROSTOMUM AMBIGUUM 427 



The first stages in the formation of the rudiments of the 

 meganucleus have not been seen. Preparation of the earHest 

 stages obtained showed two or more thin discs about the middle 

 of the exconjugant. Since these discs were denser than the 

 surrounding cytoplasm, they could be seen in living excon- 

 jugants. The ground substance of these disc-shaped rudiments 

 of the meganucleus stained very feebly, scarcely more intensely 

 than did the surrounding cytoplasm. They contained a number 

 of deeply staining granules. In the earlier stages these granules 

 were distributed through the interior of the disc, but later they 

 appeared to migrate to its periphery. Some of these granules 

 showed one or more vacuoles inside them and seemed to be 

 identical to the macrocomes of the meganucleus of normal 

 individuals of Spirostomum ambiguum (PI. 22, fig. 8, 

 M.V.). 



The normal number of these meganuclear discs present in 

 the exconjugants was two. Occasionally four, and in one 

 preparation six, were seen. Whether these large numbers 

 arose by division of an original two, or whether the normal 

 two were formed first, and later nuclei, which normally remain 

 as micronuclei, became converted into them, is not clear. 



Although I succeeded in keeping exconjugants alive in test- 

 tubes until seventeen days after the separation of the con- 

 jugants, there were no signs of constrictions appearing in the 

 rudiments of the meganucleus to form the moniliform mega- 

 nucleus. The few individuals that remained alive so long died 

 after seventeen days, and the cessation of conjugation left me 

 without any material with which to make another attempt. 



Owing to the small quantity of material at my disposal, the 

 smallness in size of the micronuclei, their great number, and 

 the difficulties of staining them whilst undergoing division, 

 my observations upon them are very fragmentary, a fault 

 which I hope to rectify in an additional paper. 



The first change in the micronuclei during conjugation 

 occurred some time after the fusion of the conjugants and 

 before the severing of the commissures of the meganuclei. 

 This change consisted of the gradual swelling up of the majority 



